Different bore/stroke within same engine
Different bore/stroke within same engine
(OP)
Can anyone determine the advantage of running two different bore/stroke combinations within the same engine?
I just read a V8 racing engine buildup in which the author said he will use two groups of 4 cyl. displacements that will be within 0.01 ci of each other.
Didn't say how cylinders would be grouped (left vs. right bank, corners vs. inner, odd vs. even).
Did say the connecting rod length-to-stroke ratio would be similar for each grouping. Any thoughts?
I just read a V8 racing engine buildup in which the author said he will use two groups of 4 cyl. displacements that will be within 0.01 ci of each other.
Didn't say how cylinders would be grouped (left vs. right bank, corners vs. inner, odd vs. even).
Did say the connecting rod length-to-stroke ratio would be similar for each grouping. Any thoughts?





RE: Different bore/stroke within same engine
Some classes check displacement by measuring one cylinder only. If you happened to make sure that only one plug was accessible, then you could make that cylinder smaller than the others (!)
The outer cylinders tend to run cooler than the inners, so again you might want to vary cylinder size for that.
Cheers
Greg Locock
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RE: Different bore/stroke within same engine
Without commenting on it's benefit or lack of, the theory is that a combination of 4 cylinders with large bore short stroke and 4 with a longer stroke and smaller bore (with all variables maximized for each 4 cylinders ie different cams ports etc) will have a longer useable power band than if you just split the difference and made them all the same. The cylinders arent always exactly the same displacement but are close. This isnt done experimentaly because it compounds the variables, It is two halfs of previously sucessful engine combinations that are fully developed. Some who are sucessful swear by it.
RE: Different bore/stroke within same engine
I wonder what effect it has on crankshaft harmonics if the strokes are different. I wonder what effect different weight rods and pistons and different piston speeds has on balance.
I know a SBC has charge robbing problems when two adjacent cylinders fire consecutively. This is exaggerated by the SBC port layout and firing order.
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RE: Different bore/stroke within same engine
Often it is just the cam specs, port specs and piston dimensions that are different.
RE: Different bore/stroke within same engine
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RE: Different bore/stroke within same engine
As to varying the cam timing, I use an APT "scatter pattern" grind on my Mini's engine. It utilizes different timing events for each cylinder to help equalize the power variation caused by the abominable port layout of the BLMC "A" engine. I have also seen different CR's used in this engine to equalize exhaust temps although I do not do this myself. I use a rather mild 13.87:1 across the board.
Rod
RE: Different bore/stroke within same engine
RE: Different bore/stroke within same engine
3rd post from the top... http://www
Related discussion...
ht
RE: Different bore/stroke within same engine
RE: Different bore/stroke within same engine
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RE: Different bore/stroke within same engine
Their dyno results and most (but not all) of the engine builders interviewed seemed to validate the conclusion that can be drawn from the old power formula.
HP=(bmep)(stroke)(Piston area)(power cycles/min)/constant
RE: Different bore/stroke within same engine
There can be very minor differences in the breathing due to rod to stroke ratio differences.
Depending on the airflow in the cylinder heads, the longer stroke might be power limited before the shorter stroke as it will reach maximum piston speed at lower rpm
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RE: Different bore/stroke within same engine
I looked at the link mentioned by incandescent, then called a friend who builds Pro Stock engines. He said he has heard of this theory but never tried it nor has he talked with anyone who has tried it. He said the theory is to build a V4 with strong top end horsepower and a V4 with more torque. The theory is that that a Pro Stock shifts around 9600 and drops about 1000 or 1100 rpm with each shift. Instead of building a V8 as a compromise in that rpm range, mate two smaller engines specific to each different rpm.
I understand the concept of manipulating the ports, ignition and cam timing and even the compression ratio to equalize cylinder performance, especially in a single 4V application. If I remember Smokey's wisdom correctly, he suggested each cylinder should be tuned to match the others in the pursuit of 8-cylinder harmony. But I had never heard of manipulating those same dynamics to split the engine's personality as a means to run better at two different rpm. My gut instinct says the two setups will conflict instead of complement. It makes me think of running an off-road vehicle with two shocks: one for small bumps and one for large berms. I don't think it'll work.
Am I just not thinking outside the box enough?
RE: Different bore/stroke within same engine
If I needed to widen a torque band, I would be more inclined to spread the cam lobe centres as the first step.
I would certainly be prepared to optimise every cylinder rather than equalise every cylinder. Equalising implies bringing all back to the lowest common denominator.
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RE: Different bore/stroke within same engine
RE: Different bore/stroke within same engine
See on a McLaren M8C:
http://im
Sorry for ridiculously long URL!
Al Seim
RE: Different bore/stroke within same engine
RE: Different bore/stroke within same engine
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RE: Different bore/stroke within same engine
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RE: Different bore/stroke within same engine
I did dimly remember the long and short ports, are they really as much different as the intake trumpet lengths in that photo?
Some of the engine development was done here in Richmond, VA USA at the former Reynolds Product Development Lab, I do know a guy who worked there, maybe he remembers something...
Regards,
Al Seim
RE: Different bore/stroke within same engine
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